Electrolyte including an additive for rechargeable lithium battery and rechargeable lithium battery including same

ABSTRACT

An electrolyte for a rechargeable lithium battery includes a non-aqueous organic solvent; a lithium salt; and an additive including vinylene carbonate, fluoroethylene carbonate, and a nitrile-based compound represented by Formula 1: 
     
       
         
         
             
             
         
       
     
     wherein n ranges from 1 to 12 and R 1  and R 2  are independently a halogen, a hydrogen, or an alkyl group. Further, the alkyl group can be C m H (2m+1) , in which m ranges from 1 to 10. The electrolyte for a rechargeable lithium battery improves storage stability of the rechargeable lithium battery at a high temperature. And, a rechargeable lithium battery including the electrolyte has improved storage stability.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims priority to and the benefit of Korean PatentApplication No. 2007-33743, filed in the Korean Intellectual PropertyOffice on Apr. 5, 2007, the entire content of which is incorporatedherein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

Aspects of the present invention relate to an electrolyte for arechargeable lithium battery and a rechargeable lithium batteryincluding the same. More particularly, the present invention relates toan electrolyte for a rechargeable lithium battery having excellentstorage stability at a high temperature, and a rechargeable lithiumbattery including the same.

2. Description of the Related Art

A lithium rechargeable battery has recently drawn attention as a powersource of a small portable electronic device. Such a secondary batteryuses an organic electrolyte solution to achieve a discharge voltagetwice as high as a conventional battery using an alkali aqueoussolution, and accordingly has a high energy density. For a positiveactive materials of a rechargeable lithium battery, lithium-transitionelement composite oxides being capable of intercalating lithium, such asLiCoO₂, LiMn₂O₄, LiNiO₂, LiNi_(1-x)Co_(x)O₂ (0<x<1), LiMnO₂, etc., havebeen researched. As for a negative active material of a rechargeablelithium battery, various carbon-based materials, such as artificial andnatural graphite, and hard carbon, have been used, which may allintercalate and deintercalate lithium ions. Positive and negativeelectrodes of a rechargeable lithium battery may be unstable dependingon a charge state at a temperature of 25° C. or more and may therebyinduce decomposition of a salt of the electrolyte, an organic solvent,and the positive and negative active materials. This decompositioncauses serious deterioration of battery stability and safety. In orderto counter such decomposition, an electrolyte including 0.001 to 0.1mol/L of an organic compound having at least two cyano groups has beensuggested. However, sufficient stability and safety have not beenrealized. High-capacity batteries are required to meet the demands ofcustomers, while high-level stability and safety are also required, andit is difficult to satisfy both requirements.

SUMMARY OF THE INVENTION

An embodiment of the present invention provides an electrolyte for arechargeable lithium battery that improves storage stability of therechargeable lithium battery at a high temperature. Another embodimentof the present invention provides a rechargeable lithium batteryincluding the electrolyte.

According to an embodiment of the present invention, provided is anelectrolyte for a rechargeable, or secondary, lithium battery thatincludes a non-aqueous organic solvent; a lithium salt; and an additiveincluding vinylene carbonate, fluoroethylene carbonate, and anitrile-based compound represented by Formula 1:

wherein n ranges from 1 to 12 and R1 and R2 are independently a halogen,a hydrogen, or an alkyl group or any other obvious variant. The alkylgroup may be described as C_(m)H_((2m+1)) in which m ranges from 1 to10.

The vinylene carbonate is present in an amount of 0.01 to 9 wt % basedon the total weight of the electrolyte. According to another embodiment,the vinylene carbonate is present in an amount of 0.5 to 5 wt % based onthe total weight of the electrolyte.

The fluoroethylene carbonate is present in an amount of 0.1 to 7 wt %based on the total weight of the electrolyte. According to anotherembodiment, the fluoroethylene carbonate is present in an amount of 0.5to 5 wt % based on the total weight of the electrolyte.

The nitrile-based compound of the above Formula 1 is present in anamount of 0.005 to 10 wt % based on the total weight of the electrolyte.According to another embodiment, the compound is present in an amount of0.01 to 5 wt % based on the total weight of the electrolyte.

The nitrile-based compound may be selected from the group consisting ofsuccinonitrile, glutaronitrile, adiponitrile, pimelonitrile,suberonitrile, and combinations thereof. According to anotherembodiment, the nitrile-based compound may be selected from the groupconsisting of, glutaronitrile, adiponitrile, pimelonitrile,suberonitrile, and combinations thereof. According to yet anotherembodiment, the nitrile-based compound may be adiponitrile.

According to an embodiment of the present invention, provided is arechargeable lithium battery including a positive electrode, a negativeelectrode, and the above electrolyte.

The positive electrode of the rechargeable lithium battery has an activemass density ranging from 3.7 g/cc to 4.2 g/cc and a charging cut-offvoltage ranging from 4.3 to 4.5V. The rechargeable lithium battery maybe charged at a high voltage.

The positive active material may be selected from the group consistingof compounds represented by Formulas 2 to 12 and mixtures thereof.Li_(a1)Ni_(b1)Co_(c1)M1_(d1)O₂  Formula 2

wherein, 0.95≦a1≦1.1, 0≦b1≦0.9, 0≦c1≦0.5, and 0≦d1≦0.2, and

M1 is selected from the group consisting of Mg, Ca, Sr, Ba, Ra, Sc, Y,Ti, Zr, Hf, Rf, V, Nb, Ta, Db, Cr, Mo, W, Sg, Tc, Re, Bh, Fe, Ru, Os,Hs, Rh, Ir, Pd, Pt, Cu, Ag, Au, Zn, Cd, B, Al, Ga, In, Tl, Si, Ge, Sn,P, As, Sb, Bi, S, Se, Te, Po, and combinations thereof. Further, d1 maybe 0.001≦d1≦0.2.Li_(a2)Ni_(b2)Co_(c2)Mn_(d2)M2_(e1)O₂  Formula 3

wherein 0.95≦a2≦1.1, 0≦b2≦0.9, 0≦c2≦0.5, 0≦d2≦0.5, and 0≦e1≦0.2, and

M2 is selected from the group consisting of Mg, Ca, Sr, Ba, Ra, Sc, Y,Ti, Zr, Hf, Rf, V, Nb, Ta, Db, Cr, Mo, W, Sg, Tc, Re, Bh, Fe, Ru, Os,Hs, Rh, Ir, Pd, Pt, Cu, Ag, Au, Zn, Cd, B, Al, Ga, In, Tl, Si, Ge, Sn,P, As, Sb, Bi, S, Se, Te, Po, and combinations thereof. Further, e1 maybe 0.001≦e1≦0.2.Li_(a3)Mn₂M3_(b3)O₄  Formula 4

wherein 0.95≦a3≦1.1, and 0≦b3≦0.2, and

M3 is selected from the group consisting of Mg, Ca, Sr, Ba, Ra, Sc, Y,Ti, Zr, Hf, Rf, V, Nb, Ta, Db, Cr, Mo, W, Sg, Tc, Re, Bh, Fe, Ru, Os,Hs, Rh, Ir, Pd, Pt, Cu, Ag, Au, Zn, Cd, B, Al, Ga, In, Tl, Si, Ge, Sn,P, As, Sb, Bi, S, Se, Te, Po, and combinations thereof. Further, b3 maybe 0.001≦b3≦0.2.GS₂  Formula 5

wherein G is Ti or Mo.LiJS₂  Formula 6

wherein J is Ti or Mo.V₂O₅  Formula 7LiV₂O₅  Formula 8LiTO₂  Formula 9

wherein T is selected from the group consisting of Cr, V, Fe, Ti, Sc, Y,and combinations thereof.LiNiVO₄  Formula 10Li_((3-a4))M′₂(PO₄)₃  Formula 11

wherein 0≦a4≦3, M′ is selected from the group consisting of V, Cr, Mn,Co, Ni, Cu, and combinations thereof.Li_((3-a′))Fe₂(PO₄)₃  Formula 12

wherein 0≦a≦2.

In the rechargeable lithium battery, the negative active material mayinclude at least one selected from the group consisting of acarbonaceous material, a lithium metal, a lithium alloy, a materialbeing capable of forming a lithium-containing compound, and combinationsthereof. According to one embodiment, the carbonaceous material may beappropriate for the negative active material. The carbonaceous materialhas an Lc (crystallite size) of at least 10 nm, and exhibits anexothermic peak at 700° C. or more.

The crystalline carbon may be a carbon prepared by carbonizing mesophasespherical particles and performing a graphitizing operation on thecarbonized material. Further, the carbonaceous material may be agraphite fiber prepared by carbonizing a mesophase pitch fiber andperforming a graphitizing operation on the carbonized material.

Additional aspects and/or advantages of the invention will be set forthin part in the description which follows and, in part, will be obviousfrom the description, or may be learned by practice of the invention.

BRIEF DESCRIPTION OF THE DRAWING

These and/or other aspects and advantages of the invention will becomeapparent and more readily appreciated from the following description ofthe embodiments, taken in conjunction with the accompanying drawing ofwhich:

FIG. 1 is a schematic cross-sectional view of a rechargeable lithiumbattery.

DETAILED DESCRIPTION OF THE EMBODIMENTS

Reference will now be made in detail to the present embodiments of thepresent invention, examples of which are illustrated in the accompanyingdrawings, wherein like reference numerals refer to the like elementsthroughout. The exemplary embodiments are described below in order toexplain aspects of the present invention.

FIG. 1 is a schematic cross-sectional view of a rechargeable lithiumbattery according to an embodiment of the present invention. Referringto FIG. 1, the rechargeable lithium battery 1 is constructed of anegative electrode 2, a positive electrode 3, a separator 4 disposedbetween the positive electrode 3 and the negative electrode 2. Therechargeable lithium battery 1 also includes a cell case 5 and a sealingmember 6 sealing the cell case 5. An electrolyte fills the cell case 5and is sealed therein by the sealing member 6 so as to electricallyconnect the negative electrode 2 and the positive electrode 3.

Aspects of the present invention relate to an electrolyte for arechargeable lithium battery that has excellent storage stability at ahigh temperature. The electrolyte for a rechargeable lithium batteryincludes a non-aqueous organic solvent, a lithium salt, and an additive.The additive includes vinylene carbonate, fluoroethylene carbonate, anda nitrile-based compound represented by Formula 1:

wherein n ranges from 1 to 12 and R₁ and R₂ are independently a halogen,a hydrogen, or an alkyl group or any other obvious variant. The alkylgroup can be described as C_(m)H_((2m+1)) in which m ranges from 1 to10. When n is greater than 1, R1 and R2 may repeat or R1 and R2 may beformed of the halogen, hydrogen, or the alkyl group and not repeat.Further, when n is greater than 1, R1 and R2 may include differenthalogens and/or alkyl groups of different lengths.

The nitrile-based compound present in the additive may be selected fromthe group consisting of succinonitrile, glutaronitrile, adiponitrile,pimelonitrile, suberonitrile, and combinations thereof. According toanother embodiment, the nitrile-based compound may be selected from thegroup consisting of, glutaronitrile, adiponitrile, pimelonitrile,suberonitrile, and combinations thereof. According to yet anotherembodiment, the nitrile-based compound may be adiponitrile.

The vinylene carbonate is present in the additive at an amount of 0.01to 9 wt % based on the total weight of the electrolyte. According toanother embodiment, the vinylene carbonate is present in the additive atan amount of 0.5 to 5 wt % based on the total weight of the electrolyte.The fluoroethylene carbonate is present in the additive at an amount of0.1 to 7 wt % based on the total weight of the electrolyte. According toanother embodiment, the fluoroethylene carbonate is present in theadditive at an amount of 0.5 to 5 wt % based on the total weight of theelectrolyte. The nitrile-based compound of the above Formula 1 ispresent in the additive at an amount of 0.005 to 10 wt % based on thetotal weight of the electrolyte. According to another embodiment, thenitrile-based compound is present in the additive at an amount of 0.01to 5 wt % based on the total weight of the electrolyte.

When the amount of the vinylene carbonate, fluoroethylene carbonate, ornitrile-based compound is out of the above-described ranges, an opencircuit voltage (OCV) of the resultant battery decreases below the valueof 4.2V.

In the electrolyte, the non-aqueous organic solvent acts as a medium fortransmitting ions taking part in the electrochemical reaction of thebattery. The non-aqueous organic solvent may include a carbonate-based,ester-based, ether-based, ketone-based, alcohol-based, or aproticsolvent. Examples of the carbonate-based solvent may include dimethylcarbonate (DMC), diethyl carbonate (DEC), dipropyl carbonate (DPC),methylpropyl carbonate (MPC), ethylpropyl carbonate (EPC), methylethylcarbonate (MEC), ethylmethyl carbonate (EMC), ethylene carbonate (EC),propylene carbonate (PC), butylene carbonate (BC), etc. Examples of theester-based solvent may include n-methyl acetate, n-ethyl acetate,n-propyl acetate, dimethylacetate, methylpropionate, ethylpropionate,γ-butyrolactone, decanolide, valerolactone, mevalonolactone,caprolactone, etc. Examples of the ether-based solvent include dibutylether, tetraglyme, diglyme, dimethoxyethane, 2-methyltetrahydrofuran,tetrahydrofuran, etc. Examples of the ketone-based solvent includecyclohexanone, etc. Examples of the alcohol-based solvent include ethylalcohol, isopropyl alcohol, etc., and examples of the aprotic solventinclude nitrites, such as R—CN (wherein R is a C2 to C20 linear,branched, or cyclic hydrocarbon, a carbon chain including double bonds,an aromatic ring, or a carbon chain including ether bonds), amides suchas dimethylformamide, dioxolanes, such as 1,3-dioxolane, sulfolanes,etc. The non-aqueous organic solvent may be used singularly or in amixture. When the organic solvent is used in a mixture, a mixture ratiomay be controlled in accordance with a desirable battery performance.

The carbonate-based solvent may include a mixture of a cyclic carbonateand a linear carbonate. The cyclic carbonate and the chain carbonate aremixed together in a volume ratio of 1:1 to 1:9, and when the mixture isused as an electrolyte, the electrolyte performance may be enhanced.

In addition, the electrolyte according to aspects of the presentinvention may further include mixtures of carbonate-based solvents andaromatic hydrocarbon-based solvents. The carbonate-based solvents andthe aromatic hydrocarbon-based solvents are preferably mixed together ina volume ratio of 1:1 to 30:1.

The aromatic hydrocarbon-based organic solvent may be represented byFormula 13.

wherein R₃ to R₈ are independently selected from the group consisting ofhydrogen, a halogen, a C1 to C10 alkyl, a haloalkyl, and combinationsthereof.

The aromatic hydrocarbon-based non-aqueous organic solvent may include,but is not limited to, at least one selected from the group consistingof benzene, fluorobenzene, 1,2-difluorobenzene, 1,3-difluorobenzene,1,4-difluorobenzene, 1,2,3-trifluorobenzene, 1,2,4-trifluorobenzene,chlorobenzene, 1,2-dichlorobenzene, 1,3-dichlorobenzene,1,4-dichlorobenzene, 1,2,3-trichlorobenzene, 1,2,4-trichlorobenzene,iodobenzene, 1,2-diiodobenzene, 1,3-diiodobenzene, 1,4-diiodobenzene,1,2,3-triiodobenzene, 1,2,4-triiodobenzene, toluene, fluorotoluene,1,2-difluorotoluene, 1,3-difluorotoluene, 1,4-difluorotoluene,1,2,3-trifluorotoluene, 1,2,4-trifluorotoluene, chlorotoluene,1,2-dichlorotoluene, 1,3-dichlorotoluene, 1,4-dichlorotoluene,1,2,3-trichlorotoluene, 1,2,4-trichlorotoluene, iodotoluene,1,2-diiodotoluene, 1,3-diiodotoluene, 1,4-diiodotoluene,1,2,3-triiodotoluene, 1,2,4-triiodotoluene, xylene, and combinationsthereof.

The non-aqueous organic solvent in the electrolyte may further includean overcharge inhibition additive such as ethylene carbonate,pyrocarbonate, etc.

The lithium salt is dissolved in the non-aqueous organic solvent tosupply lithium ions in the battery. The lithium salt is the basis ofoperation of the rechargeable lithium battery, and the lithium saltfacilitates transmission of lithium ions between the positive andnegative electrodes. Non-limiting examples of the lithium salt includeat least one supporting electrolyte salt selected from the groupconsisting of LiPF₆, LiBF₄, LiSbF₆, LiAsF₆, LiCF₃SO₃, LiN(SO₂C₂F₅)₂,LiN(CF₃SO₂)₂, LiC₄F₉SO₃, LiClO₄, LiAlO₄, LiAlCl₄,LiN(C_(x)F_(2x+1)SO₂)(C_(y)F_(2y+1)SO₂) (in which x and y are positiveintegers), LiCl, LiI, lithium bisoxalate borate, and combinationsthereof. The lithium salt may be used at a 0.1 to 2.0M concentration.When the lithium salt concentration is less than 0.1M, electrolyteperformance may decrease due to low electrolyte conductivity, whereaswhen it is more than 2.0M, lithium ion mobility may be reduced due to anincrease of the viscosity of the electrolyte.

A rechargeable lithium battery according to aspects of the presentinvention may include the electrolyte as described above and as such,the resultant battery has a particularly high voltage that is charged toa cut-off voltage ranging from 4.3 to 4.5V.

A rechargeable lithium battery includes a positive electrode including apositive active material, a negative electrode including a negativeactive material, and the above-described electrolyte.

The positive active material may be selected from the group consistingof compounds represented by Formulas 2 to 12 and mixtures thereof.Li_(a1)Ni_(b1)CO_(c1)M1_(d1)O₂  Formula 2

wherein 0.95≦a1≦1.1, 0≦b1≦0.9, 0≦c1≦0.5, 0≦d1≦0.2, and

M1 is selected from the group consisting of Mg, Ca, Sr, Ba, Ra, Sc, Y,Ti, Zr, Hf, Rf, V, Nb, Ta, Db, Cr, Mo, W, Sg, Tc, Re, Bh, Fe, Ru, Os,Hs, Rh, Ir, Pd, Pt, Cu, Ag, Au, Zn, Cd, B, Al, Ga, In, Tl, Si, Ge, Sn,P, As, Sb, Bi, S, Se, Te, Po, and combinations thereof. Furthermore, d1may be 0.001≦d1≦0.2.Li_(a2)Ni_(b2)Co_(c2)Mn_(d2)M2_(e1)O₂  Formula 3

wherein 0.95≦a2≦1.1, 0≦b2≦0.9, 0≦c2≦0.5, 0≦d2≦0.5, 0≦e1≦0.2, and

M2 is selected from the group consisting of Mg, Ca, Sr, Ba, Ra, Sc, Y,Ti, Zr, Hf, Rf, V, Nb, Ta, Db, Cr, Mo, W, Sg, Tc, Re, Bh, Fe, Ru, Os,Hs, Rh, Ir, Pd, Pt, Cu, Ag, Au, Zn, Cd, B, Al, Ga, In, Tl, Si, Ge, Sn,P, As, Sb, Bi, S, Se, Te, Po, and combinations thereof. Furthermore, e1may be 0.001≦e1≦0.2.Li_(a3)Mn₂M3_(b3)O₄  Formula 4

wherein 0.95≦a3≦1.1, 0≦b3≦0.2, and

M3 is selected from the group consisting of Mg, Ca, Sr, Ba, Ra, Sc, Y,Ti, Zr, Hf, Rf, V, Nb, Ta, Db, Cr, Mo, W, Sg, Tc, Re, Bh, Fe, Ru, Os,Hs, Rh, Ir, Pd, Pt, Cu, Ag, Au, Zn, Cd, B, Al, Ga, In, Tl, Si, Ge, Sn,P, As, Sb, Bi, S, Se, Te, Po, and combinations thereof. Furthermore, b3may be 0.001≦b3≦0.2.GS₂  Formula 5

wherein G is Ti or Mo.LiJS₂  Formula 6

wherein J is Ti or Mo.V₂O₅  Formula 7LiV₂O₅  Formula 8LiTO₂  Formula 9

wherein T is selected from the group consisting of Cr, V, Fe, Ti, Sc, Y,and combinations thereof.LiNiVO₄  Formula 10Li_((3-a4))M′₂(PO₄)₃  Formula 11

wherein 0<a4<3, M′ is selected from the group consisting of V, Cr, Mn,Co, Ni, Cu, and combinations thereof.Li_((3-a′))Fe₂(PO₄)₃  Formula 12

wherein 0≦a′≦2.

The positive electrode may have an active mass density ranging from 3.7to 4.2 g/cm³. The positive electrode may include the above-describedpositive active material, a binder, a conductive agent, and a positivecurrent collector. The conductive agent may be omitted when notrequired.

The binder binds the active material particles together and also thepositive active materials to a current collector. Examples of the binderinclude, but are not limited to, polyvinylalcohol,carboxylmethylcellulose, hydroxypropylcellulose, diacetylenecellulose,polyvinylchloride, polyvinylpyrrolidone, polytetrafluoroethylene,polyvinylidene fluoride, polyethylene, or polypropylene.

Any electrically conductive material may be used as a conductive agentunless it causes any chemical change or reacts undesirably with theother constituents. Examples of the conductive agent include naturalgraphite, artificial graphite, carbon black, acetylene black, ketjenblack, a carbon fiber, a metal powder or a metal fiber including copper,nickel, aluminum, silver, etc., a polyphenylene derivative, orcombinations thereof.

The positive current collector of the positive electrode may be analuminum foil but is not limited thereto.

The negative active material in the negative electrode may include atleast one selected from the group consisting of a carbonaceous material,a lithium metal, a lithium alloy, a material being capable of forming alithium-containing compound, and combinations thereof. According to anembodiment, the carbonaceous material may be appropriate for thenegative active material.

The carbonaceous material may be amorphous carbon or crystalline carbon.The amorphous carbon may be a soft carbon (carbon obtained by sinteringat a low temperature), a hard carbon (carbon obtained by sintering at ahigh temperature), mesophase pitch carbide, fired coke, etc., and thecrystalline carbon may be non-shaped, sheet, flake, spherical, or fibershaped natural or artificial graphite.

The carbonaceous material has an Lc (crystallite size) of at least 10 nmfound through Scherrer analysis of X-ray diffraction peaks. According toone embodiment, the carbonaceous material has an Lc of 10 to 1500 nmfound through X-ray diffraction. The carbonaceous material exhibits anexothermic peak at 700° C. or more. The exothermic peak differentiatescrystalline or amorphous carbon. The exothermic peak at 700° C. or moreindicates that the carbon is crystalline carbon, and therefore, themaximum value of the exothermic temperature need not be limited.

The crystalline carbon may be a carbon prepared by carbonizing mesophasespherical particles and performing a graphitizing operation on thecarbonized material. Further, the carbonaceous material may be agraphite fiber prepared by carbonizing a mesophase pitch fiber andperforming a graphitizing operation on the carbonized material.

The lithium alloy that may be included in the negative active materialincludes lithium and a metal selected from the group consisting of Na,K, Rb, Cs, Fr, Be, Mg, Ca, Sr, Ba, Ra, Al, and Sn.

The material being capable of reversibly forming a lithium-containingcompound by reaction with lithium ions which may also be included in thenegative active material includes tin oxide (SnO₂), titanium nitrate,silicon (Si), etc., but is not limited thereto.

The negative electrode includes the above negative active material, abinder, a negative current collector, and optionally a conductive agent.The binder and conductive agent are the same as described with respectto the positive electrode and therefore their descriptions are notprovided. The negative electrode includes a negative current collector,such as a copper foil, but is not limited thereto.

The negative electrode may be fabricated as follows. The negative activematerial composition is prepared by mixing the negative active material,the binder, and optionally the conductive agent, and then applying thecomposition to the negative current collector. TAs for the solvent usedduring fabrication of the electrode, any solvent may be used. Forexample, N-methylpyrrolidone may be used, but is not limited thereto.

The rechargeable lithium battery generally includes a positiveelectrode, a negative electrode, and an electrolyte. The battery mayfurther include a separator, as needed. The separator may include anymaterial used in conventional lithium secondary batteries. Non-limitingexamples of suitable separator materials include polyethylene,polypropylene, polyvinylidene fluoride, and multi-layers thereof, suchas a polyethylene/polypropylene double-layered separator, apolyethylene/polypropylene/polyethylene triple-layered separator, or apolypropylene/polyethylene/polypropylene triple-layered separator.

Rechargeable lithium batteries may be classified as lithium ionbatteries, lithium ion polymer batteries, and lithium polymer batteriesaccording to the presence of a separator and the kind of electrolyteused in the battery. The rechargeable lithium batteries may have avariety of shapes and sizes, including cylindrical, prismatic, orcoin-type batteries, and may be a thin film battery or larger in size.

The following examples illustrate aspects of the present invention inmore detail. These examples, however, should not in any sense beinterpreted as limiting the scope of the present invention.

Example 1

An LiCoO₂ positive active material, a polyvinylidene fluoride binder,and a SuperP conductive agent were mixed in a ratio of 96:2:2 wt % in anN-methylpyrrolidone solvent to prepare a positive active materialslurry. The positive active material slurry was coated on an aluminumcurrent collector and then dried and compressed to prepare a positiveelectrode. The positive electrode had an active mass density of 3.73g/cm³.

Next, a negative active material slurry was prepared by mixing anartificial graphite negative active material and a polyvinylidenefluoride binder in a ratio 94:6 wt % in an N-methylpyrrolidone solvent.The slurry was coated on a copper current collector and then dried andcompressed to prepare a negative electrode. The graphite negative activematerial had an Lc of about 100 nm according to Scherrer analysis ofX-ray diffraction peaks, and the graphite negative active material hadan exothermic peak at more than 700° C.

Then, a 1.3M LiPF₆ electrolyte was prepared by dissolving LiPF₆ in amixed solvent of ethylene carbonate, dimethyl carbonate, and ethylmethylcarbonate, which were mixed in a volume ratio of 30:30:40. The additive,including vinylene carbonate, fluoroethylene carbonate, andsuccinonitrile, was then added thereto. The vinylene carbonate, thefluoroethylene carbonate, and the succinonitrile were respectivelyincluded in an amount of 1 wt % based on the total weight of theelectrolyte (i.e., the LiPF₆, the mixed solvent, and the additive).

A rechargeable lithium battery cell was fabricated using the abovepositive electrode, negative electrode, and electrolyte.

Example 2

A rechargeable lithium battery cell was fabricated in according to thesame method as in Example 1, except that glutaronitrile was used insteadof succinonitrile.

Example 3

A rechargeable lithium battery cell was fabricated according to the samemethod as in Example 1, except that adiponitrile was used instead ofsuccinonitrile.

Example 4

A rechargeable lithium battery cell was fabricated according to the samemethod as in Example 1, except that pimelonitrile was used instead ofsuccinonitrile.

Example 5

A rechargeable lithium battery cell was fabricated according to the samemethod as in Example 1, except that suberonitrile was used instead ofsuccinonitrile.

Comparative Example 1

A positive active material slurry was prepared by mixing an LiCoO₂positive active material, a polyvinylidene fluoride binder, and a SuperPconductive agent in a ratio of 96:2:2 wt % in an N-methylpyrrolidonesolvent. The positive active material slurry was coated on an aluminumcurrent collector and then dried and compressed to prepare a positiveelectrode. The positive electrode had active mass density of 3.73 g/cm³.

Next, a negative active material slurry was prepared by mixing anartificial graphite negative active material and a polyvinylidenefluoride binder in a ratio of 94:6 wt % in an N-methylpyrrolidonesolvent. The slurry was coated on a copper current collector and thendried and compressed to prepare a negative electrode. The graphitenegative active material had an Lc of about 100 nm according to Scherreranalysis of X-ray diffraction peaks, and the graphite negative activematerial had an exothermic peak at more than 700° C.

Then, a 1.3M LiPF₆ electrolyte was prepared by dissolving LiPF₆ in amixed solvent of ethylene carbonate, dimethyl carbonate, and ethylmethylcarbonate, which where mixed in a volume ratio of 30:30:40.

A rechargeable lithium battery cell was fabricated using the abovepositive electrode, negative electrode, and electrolyte.

Comparative Example 2

A rechargeable lithium battery cell was fabricated according to the samemethod as in Comparative Example 1, except that the electrolyte wasprepared by mixing ethylene carbonate, dimethyl carbonate, ethylmethylcarbonate in a volume ratio of 30:30:40 to prepare a mixed solvent, andthen dissolving LiPF₆ in the mixed solvent and adding vinylene carbonateto produce a 1.3M LiPF₆ electrolyte. Herein, the vinylene carbonate wasincluded in an amount of 1 wt % based on the total weight of theelectrolyte.

Comparative Example 3

A rechargeable lithium battery cell was fabricated according to the samemethod as in Comparative Example 1, except that the electrolyte wasprepared by mixing ethylene carbonate, dimethyl carbonate, andethylmethyl carbonate in a volume ratio of 30:30:40 to prepare a mixedsolvent, and then dissolving LiPF₆ in the mixed solvent and addingfluoroethylene carbonate to produce a 1.3M LiPF₆ electrolyte. Herein,the fluoroethylene carbonate was included in an amount of 1 wt % basedon the total weight of the electrolyte.

Comparative Example 4

A rechargeable lithium battery cell was fabricated according to the samemethod as in Comparative Example 1, except that the electrolyte wasprepared by mixing ethylene carbonate, dimethyl carbonate, andethylmethyl carbonate in a volume ratio of 30:30:40 to prepare a mixedsolvent, and dissolving LiPF₆ in the mixed solvent and addingadiponitrile to produce a 1.3M LiPF₆ electrolyte. Herein, theadiponitrile was included in an amount of 1 wt % based on the totalweight of the electrolyte.

Comparative Example 5

A rechargeable lithium battery cell was fabricated according to the samemethod as in Comparative Example 1, except that the electrolyte wasprepared by mixing ethylene carbonate, dimethyl carbonate, ethylmethylcarbonate in a volume ratio of 30:30:40 to prepare a mixed solvent, andthen dissolving LiPF₆ in the mixed solvent and adding vinylene carbonateand adiponitrile to produce a 1.3M LiPF₆ electrolyte. Herein, thevinylene carbonate and the adiponitrile were included in an amount of 1wt % based on the total weight of the electrolyte.

Comparative Example 6

A rechargeable lithium battery cell was fabricated according to the samemethod as in Comparative Example 1, except that the electrolyte wasprepared by mixing ethylene carbonate, dimethyl carbonate, andethylmethyl carbonate in a volume ratio of 30:30:40 to prepare a mixedsolvent, dissolving LiPF₆ in the mixed solvent and adding vinylenecarbonate and fluoroethylene to produce a 1.3M LiPF₆ electrolyte.Herein, the vinylene carbonate and the fluoroethylene were included inan amount of 1 wt % based on the total weight of the electrolyte.

Comparative Example 7

A rechargeable lithium battery cell was fabricated according to the samemethod as in Comparative Example 1, except that the electrolyte wasprepared by mixing ethylene carbonate, dimethyl carbonate, andethylmethyl carbonate in a volume ratio of 30:30:40 to prepare a mixedsolvent, dissolving LiPF₆ in the mixed solvent to prepare a 1.3M LiPF₆electrolyte, and adding fluoroethylene and adiponitrile thereto. Herein,the fluoroethylene carbonate and the adiponitrile were respectivelyincluded in an amount of 1 wt % based on the total weight of theelectrolyte.

All of the rechargeable lithium battery cells according to Examples 1 to5 and Comparative Examples 1 to 7 were charged at 0.2 C and thendischarged at 0.2 C for a formation charge and discharge, and thencharged at 0.5 C and discharged at 0.2 C for a standard charge anddischarge. Subsequently, the cells were charged at 0.2 C to 4.35V. Therechargeable lithium batteries were all formed as 18650 cylindricalcells.

The battery cells were allowed to stand at 60° C., and after two weekstheir OCVs were measured. The measurement results are shown in thefollowing Table 1. In the following Table 1, SN denotes succinonitrile,GN denotes glutaronitrile, AN denotes adiponitrile, PN denotespimelonitrile, and UN denotes suberonitrile. Further, a “y” indicatesthat the component listed was present in the example, and an “n”indicates that the component was not included in the example.

TABLE 1 Vinylene Fluoroethylene Nitrile-based OCV after 2 carbonatecarbonate compound weeks (V) Comparative n n n 4.03 Example 1Comparative y n n 4.08 Example 2 Comparative n y n 4.05 Example 3Comparative n n AN 4.10 Example 4 Comparative y n AN 4.12 Example 5Comparative y y n 4.15 Example 6 Comparative n y AN 4.13 Example 7Example 1 y y SN 4.28 Example 2 y y GN 4.30 Example 3 y y AN 4.32Example 4 y y PN 4.33 Example 5 y y UN 4.32

As shown in Table 1, the cells fabricated by using the electrolyteincluding vinylene carbonate, fluoroethylene carbonate, and anitrile-based compound according to Examples 1 to 5 exhibited an OCV of4.2V 2 weeks later, which indicates that the battery maintained thedesired standard charge after having been stored at 60° C. for the2-week period. In addition, among the nitrile-based compounds,glutaronitrile, adiponitrile, pimelonitrile, and suberonitrile may bepreferable to succinonitrile. In contrast, the cells missing at leastone of the compounds had OCVs of less than 4.2V.

Further examples were made to demonstrate the effect of weight percentof adiponitrile added to the electrolyte. The examples were based onExample 3 as described above.

Example 6

A rechargeable lithium battery cell was fabricated according to the samemethod as in Example 3, except that the amount of adiponitrile waschanged from 1 wt % to 0.005 wt % based on the total weight of theelectrolyte.

Example 7

A rechargeable lithium battery cell was fabricated according to the samemethod as in Example 3, except that the amount of adiponitrile waschanged from 1 wt % to 0.01 wt % based on the total weight of theelectrolyte.

Example 8

A rechargeable lithium battery cell was fabricated according to the samemethod as in Example 3, except that the amount of adiponitrile waschanged from 1 wt % to 0.025 wt % based on the total weight of theelectrolyte.

Example 9

A rechargeable lithium battery cell was fabricated according to the samemethod as in Example 3, except that the amount of adiponitrile waschanged from 1 wt % to 0.05 wt % based on the total weight of theelectrolyte.

Example 10

A rechargeable lithium battery cell was fabricated according to the samemethod as in Example 3, except that the amount of adiponitrile waschanged from 1 wt % to 2 wt % based on the total weight of theelectrolyte.

Example 11

A rechargeable lithium battery cell was fabricated according to the samemethod as in Example 3, except that the amount of adiponitrile waschanged from 1 wt % to 5 wt % based on the total weight of theelectrolyte.

Example 12

A rechargeable lithium battery cell was fabricated according to the samemethod as in Example 3, except that the amount of adiponitrile waschanged from 1 wt % to 10 wt % based on the total weight of theelectrolyte.

Comparative Example 8

A rechargeable lithium battery cell was fabricated according to the samemethod as in Example 3, except that the amount of adiponitrile waschanged from 1 wt % to 0.001 wt % based on the total weight of theelectrolyte.

Comparative Example 9

A rechargeable lithium battery cell was fabricated according to the samemethod as in Example 3 except that the amount of adiponitrile waschanged from 1 wt % to 20 wt % based on the total weight of theelectrolyte.

The 18650 cylindrical rechargeable lithium battery cells fabricatedaccording to Examples 6 to 12 and Comparative Examples 8 and 9 werecharged at 0.2 C and then discharged at 0.2 C for a formation charge anddischarge, and then charged at 0.5 C and discharged at 0.2 C for astandard charge and discharge. Subsequently, the cells were charged at0.2 C to 4.35V.

The battery cells were allowed to stand at 60° C., and after two weeks,their OCVs were measured. The measurement results are shown in thefollowing Table 2. For comparison, the measurement results of Example 3and Comparative Example 6 are also shown in the following Table 2.

TABLE 2 Adiponitrile OCV after content (wt %) 2 weeks (V) ComparativeExample 6 0 4.15 Comparative Example 8 0.001 4.19 Example 6 0.005 4.21Example 7 0.01 4.25 Example 8 0.025 4.26 Example 9 0.05 4.28 Example 3 14.32 Example 10 2 4.31 Example 11 5 4.30 Example 12 10 4.21 ComparativeExample 9 20 4.11

As shown in Table 2, when an electrolyte including adiponitrile in anamount of 0.005 to 10 wt % was used to fabricate the cells according toExamples 3 and 6 to 12, the cells had OCVs of more than 4.2V afterhaving been stored at 60° C. for 2 weeks, which meets the standardsnecessary for the battery. However, when adiponitrile is included at anamount outside the specified range as demonstrated by ComparativeExamples 6, 8, and 9, the cells had OCVs of less than 4.2V. Accordingly,adiponitrile may be included in the electrolyte at an amount rangingfrom 0.005 to 10 wt %.

Further examples were made to illustrate the effects of the amount ofvinylene carbonate on the OCV after storage at 60° C. for two weeks. Theexamples were based again on the Example 3 as described above.

Example 13

A rechargeable lithium battery cell was fabricated according to the samemethod as in Example 3, except that the amount of vinylene carbonate waschanged from 1 wt % to 0.01 wt % based on the total weight of theelectrolyte.

Example 14

A rechargeable lithium battery cell was fabricated according to the samemethod as in Example 3, except that the amount of vinylene carbonate waschanged from 1 wt % to 0.5 wt % based on the total weight of theelectrolyte.

Example 15

A rechargeable lithium battery cell was fabricated according to the samemethod as in Example 3, except that the amount of vinylene carbonate waschanged from 1 wt % to 2 wt % based on the total weight of theelectrolyte.

Example 16

A rechargeable lithium battery cell was fabricated according to the samemethod as in Example 3, except that the amount of vinylene carbonate waschanged from 1 wt % to 3 wt % based on the total weight of theelectrolyte.

Example 17

A rechargeable lithium battery cell was fabricated according to the samemethod as in Example 3, except that the amount of vinylene carbonate waschanged from 1 wt % to 5 wt % based on the total weight of theelectrolyte.

Example 18

A rechargeable lithium battery cell was fabricated according to the samemethod as in Example 3, except that the amount of vinylene carbonate waschanged from 1 wt % to 7 wt % based on the total weight of theelectrolyte.

Example 19

A rechargeable lithium battery cell was fabricated according to the samemethod as in Example 3, except that the amount of vinylene carbonate waschanged from 1 wt % to 9 wt % based on the total weight of theelectrolyte.

Comparative Example 10

A rechargeable lithium battery cell was fabricated according to the samemethod as in Example 3, except that the amount of vinylene carbonate waschanged from 1 wt % to 10 wt % based on the total weight of theelectrolyte.

Comparative Example 11

A rechargeable lithium battery cell was fabricated according to the samemethod as in Example 3, except that the amount of vinylene carbonate waschanged from 1 wt % to 20 wt % based on the total weight of theelectrolyte.

The 18650 cylindrical rechargeable lithium battery cells according toExamples 13 to 19 and Comparative Examples 10 and 11 were charged at 0.2C and then discharged at 0.2 C for a formation charge and discharge, andthen charged at 0.5 C and discharged at 0.2 C for a standard charge anddischarge. Subsequently, the cells were charged at 0.2 C to 4.35V.

The battery cells were allowed to stand at 60° C., and after two weeks,their OCVs were measured. The measurement results are shown in thefollowing Table 3. For comparison, the measurement results of Example 3and Comparative Example 7 are also shown in the following Table 3.

TABLE 3 Vinylene carbonate OCV after 2 content (wt %) weeks (V)Comparative Example 7 0 4.13 Example 13 0.01 4.20 Example 14 0.5 4.25Example 3 1 4.32 Example 15 2 4.33 Example 16 3 4.31 Example 17 5 4.27Example 18 7 4.25 Example 19 9 4.22 Comparative Example 10 10 4.18Comparative Example 11 20 4.10

As shown in Table 3, the cells fabricated with an electrolyte includingvinylene carbonate in an amount of 0.01 to 9 wt % according to Examples3 and 13 to 19 all had OCVs of more than 4.2V after two weeks of storageat 60° C. However, when vinylene carbonate is included at an amountoutside the specified range as demonstrated by Comparative Examples 7,10, and 11, the cells had OCVs of less than 4.2V. Accordingly, vinylenecarbonate may be included in an amount ranging from 0.01 to 9 wt %.

Further examples were made to demonstrate the effect of changing theamount of fluoroethylene carbonate on the OCV after storage for 2 weeksat 60° C. The below examples were based again on Example 3 as describedabove.

Example 20

A rechargeable lithium battery cell was fabricated according to the samemethod as in Example 3, except that the amount of fluoroethylenecarbonate was changed from 1 wt % to 0.1 wt % based on the total weightof the electrolyte.

Example 21

A rechargeable lithium battery cell was fabricated according to the samemethod as in Example 3, except that the amount of fluoroethylenecarbonate was changed from 1 wt % to 0.5 wt % based on the total weightof the electrolyte.

Example 22

A rechargeable lithium battery cell was fabricated according to the samemethod as in Example 3, except that the amount of fluoroethylenecarbonate was changed from 1 wt % to 3 wt % based on the total weight ofthe electrolyte.

Example 23

A rechargeable lithium battery cell was fabricated according to the samemethod as in Example 3, except that the amount of fluoroethylenecarbonate was changed from 1 wt % to 5 wt % based on the total weight ofthe electrolyte.

Example 24

A rechargeable lithium battery cell was fabricated according to the samemethod as in Example 3, except that the amount of fluoroethylenecarbonate was changed from 1 wt % to 7 wt % based on the total weight ofthe electrolyte.

Comparative Example 12

A rechargeable lithium battery cell was fabricated according to the samemethod as in Example 3, except that the amount of fluoroethylenecarbonate was changed from 1 wt % to 0.01 wt % based on the total weightof the electrolyte.

Comparative Example 13

A rechargeable lithium battery cell was fabricated according to the samemethod as in Example 3, except that the amount of fluoroethylenecarbonate was changed from 1 wt % to 0.05 wt % based on the total weightof the electrolyte.

Comparative Example 14

A rechargeable lithium battery cell was fabricated according to the samemethod as in Example 3, except that the amount of fluoroethylenecarbonate was changed from 1 wt % to 9 wt % based on the total weight ofthe electrolyte.

Comparative Example 15

A rechargeable lithium battery cell was fabricated according to the samemethod as in Example 3, except that the amount of fluoroethylenecarbonate was changed from 1 wt % to 10 wt % based on the total weightof the electrolyte.

Comparative Example 16

A rechargeable lithium battery cell was fabricated according to the samemethod as in Example 3, except that the amount of fluoroethylenecarbonate was changed from 1 wt % to 11 wt % based on the total weightof the electrolyte.

The 18650 cylindrical rechargeable lithium battery cells according toExamples 20 to 24 and Comparative Examples 12 to 16 were charged at 0.2C and then discharged at 0.2 C for a formation charge and discharge, andthen charged at 0.5 C and discharged at 0.2 C for a standard charge anddischarge. Subsequently, the cells were charged at 0.2 C to 4.35V.

The battery cells were allowed to stand at 60° C., and after two weeks,their OCVs were measured. The measurement results are shown in thefollowing Table 4. For comparison, the measurement results of Example 3and Comparative Example 4 are also shown in the following Table 4.

TABLE 4 Fluoroethylene carbonate OCV after 2 content (wt %) weeks (V)Comparative Example 4 0 4.12 Comparative Example 12 0.01 4.17Comparative Example 13 0.05 4.19 Example 20 0.1 4.20 Example 21 0.5 4.24Example 3 1 4.32 Example 22 3 4.28 Example 23 5 4.27 Example 24 7 4.22Comparative Example 14 9 4.19 Comparative Example 15 10 4.18 ComparativeExample 16 11 4.11

As shown in Table 4, the cells fabricated with an electrolyte including0.1 to 7 wt % of fluoroethylene carbonate according to Examples 3 and 20to 24 had OCVs of 4.2V after storage at 60° C. for 2 weeks. However,when fluoroethylene carbonate is included at an amount outside thespecified range as demonstrated by Comparative Examples 4, and 12 to 16,the cells had OCVs of less than 4.2V. Accordingly, fluoroethylenecarbonate may be included in an amount ranging from 0.1 to 7 wt %.

From the results in Tables 1 to 4, when vinylene carbonate was includedat an amount ranging from 0.01 to 9 wt %, fluoroethylene carbonate wasincluded at an amount ranging from 0.1 to 7 wt %, and a nitrile-basedcompound, such as adiponitrile, was included at an amount ranging from0.005 to 10 wt % to prepare an electrolyte, a cell including theelectrolyte demonstrated excellent storage stability at a hightemperature.

As described above, an electrolyte according to aspects of the presentinvention provides a rechargeable lithium battery with excellent storagestability at a high temperature. Although a few exemplary embodiments ofthe present invention have been shown and described, it would beappreciated by those skilled in the art that changes may be made inthese embodiments without departing from the principles and spirit ofthe invention, the scope of which is defined in the claims and theirequivalents.

What is claimed is:
 1. An electrolyte for a rechargeable lithiumbattery, comprising: a non-aqueous organic solvent; a lithium salt; andan additive including vinylene carbonate, fluoroethylene carbonate, anda nitrile-based compound selected from the group consisting ofsuccinonitrile, glutaronitrile, adiponitrile, pimelonitrile,suberonitrile, and combinations thereof, wherein the vinylene carbonateis present at an amount of 0.01 to 9 wt % based on the total weight ofthe electrolyte, wherein the fluoroethylene carbonate is present at anamount of 0.1 to 7 wt % based on total weight of the electrolyte, andwherein the nitrile-based compound is present at an amount of 0.005 to10 wt % based on the total weight of the electrolyte.
 2. The electrolyteof claim 1, wherein the vinylene carbonate is present at an amount of0.5 to 5 wt % based on the total weight of the electrolyte.
 3. Theelectrolyte of claim 1, wherein the fluoroethylene carbonate is presentin an amount of 0.5 to 5 wt % based on the total weight of theelectrolyte.
 4. The electrolyte of claim 1, wherein the nitrile-basedcompound of the above Formula 1 is present at an amount of 0.01 to 5 wt% based on the total weight of the electrolyte.
 5. The electrolyte ofclaim 1, wherein the nitrile-based compound is selected from the groupconsisting of glutaronitrile, adiponitrile, pimelonitrile,suberonitrile, and combinations thereof.
 6. The electrolyte of claim 5,wherein the nitrile-based compound is adiponitrile.
 7. The electrolyteof claim 1, wherein the non-aqueous organic solvent is selected from thegroup consisting of a carbonate-based solvent, an ester-based solvent,an ether-based solvent, a ketone-based solvent, an alcohol-basedsolvent, an aprotic solvent, and combinations thereof.
 8. Theelectrolyte of claim 1, wherein the lithium salt is one selected fromthe group consisting of LiPF₆, LiBF₄, LiSbF₆, LiAsF₆, LiCF₃SO₃,LiN(SO₂C₂F₅)₂, LiN(CF₃SO₂)₂, LiC₄F₉SO₃, LiClO₄, LiAlO₄, LiAlCl₄, LiCl,LiI, lithium bisoxalate borate, and combinations thereof.
 9. Theelectrolyte of claim 1, wherein the lithium salt is present in theelectrolyte at a 0.1 to 2.0M concentration.
 10. The electrolyte of claim1, wherein the non-aqueous organic solvent comprises a mixture of acarbonate-based solvent and an aromatic hydrocarbon-based solvent. 11.The electrolyte of claim 10, wherein the mixture has a volume ratio ofabout 1:1 to 30:1 of the carbonate-based solvent to the aromatichydrocarbon-based solvent.
 12. The electrolyte of claim 1, wherein thenon-aqueous organic solvent comprises an overcharge inhibition additive.13. The electrolyte of claim 7, wherein the carbonate-based solventcomprises a mixture of a cyclic carbonate and a linear carbonate. 14.The electrolyte of claim 13, wherein the mixture comprises a volumeratio of about 1:1 to 1:9 of the cyclic carbonate to the linearcarbonate.